Process of producing iron-chromium castings containing nitrogen



Patented Oct. 6, 1933 PROCESS OF PRODUCING IRON-CHROMIUM CASTINGS CONTAINING NITROGEN 7 Frederick M.'Becket,'New York, N. Y., assignor, by mesne assignments, to Union Carbide and Carbon Corporation, a corporation oi! New York No Drawing. Application August 14, 1934, Serial No. 739,768

2 Claims.

The invention is a process of producing sound, fine-grained castings of an alloy containing as essential ingredients chromium, iron, and nitrogen.

Until recently, nitrogen was considered to'be an undesirable impurity in steels and every effort was made to provide nitrogen-free prealloys for use in making steel. For example, the nitrogen content of commercial low carbon ferrochromium was rarely permitted to exceed about 0.2%. It is now known that certain steels, notably the stainless varieties of chromium steels and chromium nickel steels, are benefited for many purposes by the addition of substantial amounts of nitrogen.

Chromium has a strong aflinity for nitrogen, and will combine stably with far more nitrogen than will iron. The affinity of iron-chromium alloys for nitrogen depends primarily on their chromium content, the alloys richer in chromium having a greater capacity for stably combined nitrogen than the alloys leaner in chromium. Therefore, if pure chromium nitrides or a ferrochromium alloy saturated with nitrogen is diluted with iron to form steel, gaseous nitrogen is evolved with the result that sound ingots can be obtained, if at all, only with difliculty.

Furthermore, the higher nitrides of chromium decompose at temperatures attained in a steel melting furnace or in a steel bath, and this decomposition further complicates the problem of obtaining sound castings of iron-chromium-nitrogen stainless steels.

The above-described diiiiculties in the manufacture of nitrogen-containing chromium steels are avoided if the nitrogen and chromium contents of the steel are added simultaneously as a high-nitrogen ferrochromium having nitrogen and chromium contents in a suitable ratio. More specifically, the weight ratio of nitrogen to chromium should be approximately 1:100. If this ratio is very much smaller than 1:100, say as small as 1:200, there will be insuflicient nitrogen in the steel to impart appreciable benefits;

for this reason, the ratio should be at least 1:175 and preferably at least 1:150. On the other hand, if this ratio is very much larger than 1:100, say 1:50, the above described dimculties in obtaining sound castings are encountered; hence,

the ratio should be no larger than about 1:75.

The ferrochromium prealloy for use in the;

process of the invention may be made by any of a number of methods. For example, chromium and/or iron nitrides may be added to molten low carbon ferrochromium; or air, nitrogen, or

ammonia may be blown through a bath of molten low-carbon ferrochrome. Another suitable process comprises reacting silicon and a nitrate in the presence of molten ierrochromium, as described in Patent 2,027,837, issued January 14, 1936, to William C. Read.

The invention is the process of producing sound, fine-grained chromium-iron castings which comprises adding to a bath of iron or steel a prealloy comprising 60% to chromium and 0.35% to 1% nitrogen, the ratio of nitrogen to chromium being between 1:175 and 1:75 and preferably 1:100.v Other elements, such as nickel for example, may be added if desired.

I claim:

1. The proces of producing sound, finegrained chromium-iron castings which comprises forming a melt containing iron; adding to the said melt an "iron-chromium-nitrogen prealloy containing about. 60% to 80% chromium and 0.35% to 1% nitrogen, the ratio of nitrogen to chromium in said prealloy being between about 1:175 and 1:75; and then casting the melt.

2. The process of producing sound, tinegrained chromium-iron castings which comprises forming a melt containing iron; adding to the said melt aniron-chromium-nitrogen prealloy containing about 60% to 80% chromium and 0.35% to 1% nitrogen, the ratio of nitrogen to chromium in said prealloy being approximately 1: and then casting the melt.

FREDERICK 'M. BECKE'I. 

